Recent study of Galileo images may present the strongest evidence to
date for understanding the strange surface tectonics of Jupiter’s moon
Europa. One of the most striking features of Europa is its fracture-dominated,
water-ice surface. But since planetary scientists began studying the jigsawed
moon, it appeared devoid of any signs of compressional features, such as
folds, typical of those on Earth. Now, researchers have found evidence
for large-scale folding in a region of Europa that includes the well-studied
Astypalaea Linea fault.

“The apparent lack of compression on Europa has puzzled researchers
since Voyager days,” says Louise Prockter of The Johns Hopkins University
Applied Physics Laboratory. Explanations for the moon’s lack of compressional
features have ranged from subduction mechanisms that escaped detection
to a theory that Europa was expanding. Prockter and Robert Pappalardo of
Brown University presented a new analysis of Galileo data suggesting that
Europan folds do exist. Their March 16 announcement at the 31st Lunar and
Planetary Science Conference in Houston has created “an element of excitement
that compressional features have finally been identified,” Prockter says.

Astypalaea
Linea fault is similar in size to the California
segment of the San Andreas fault.
Fold structures appear in the lower region
of the image. NASA/JPL.

Their hypothesis, they say, supports the theory that Europa’s crust
is deformed by gravitational stresses that result from the moon’s proximity
to Jupiter. As Europa travels around its own axis and around Jupiter, the
huge planet’s gravity exerts a continuous pull. Much like the interaction
of Earth’s moon on the ebb and flow of the tides, the jovian gravitational
stresses appear to result in the flexing of Europa. The axes of the fold
structures in the Astypalaea Linea region are oriented perpendicular to
the maximum compressional stresses predicted from the gravity deformation
models. Therefore, if Jupiter continually exerts a deformational force
on Europa, the newly discovered folds are oriented exactly as pre-existing
deformation models would predict.

Galileo collected the images at a resolution of 43 meters per
pixel, revealing to the researchers fine-scale features that are often
associated with fold structures on Earth. For example, joints can form
along the crests of terrestrial folds, and these are seen on Europa as
sets of smaller fractures striking along the crests of the regional-scale
anticlines. The researchers observed compressional ridges two to three
kilometers long within the synclinal lows. The small ridges are akin to
the bunching observed in troughs composed of pliable materials on Earth.

By measuring shaded textures on the Galileo images, Prockter and Pappalardo
could distinguish adjacent anticlines and synclines with distances between
crests of approximately 25 kilometers. Combining their measurements of
fold wavelengths with their knowledge of the physical properties of Europa’s
icy crust, Prockter and Pappalardo were also able to calculate a regional
crustal thickness of two kilometers around Astypalaea Linea. The two-kilometer
depth correlated well with the results of several pre-existing crustal
models.

In the fall of 1998, Patricio Figueredo of Arizona State University
suggested that undulations in another region of Europa may also have been
caused by gravity-driven compressional stresses. “The wavelength of the
undulations,” Figueredo says, “is about 25 kilometers — similar to that
reported by Prockter and others.” Figueredo is teaming up with Prockter
and Pappalardo to search for new sites that may show fold evidence.

According to Figueredo, the research may color future missions to Europa.
Subsequent data may be collected while the sun is at a low angle, enhancing
the shadowing that led to the researchers’ discovery of the folds.

Galileo, already operating almost three years beyond its original
completion date, will continue to observe Jupiter at least through the
end of this year. Europa may not get its next observation until 2003, the
year NASA has proposed sending a new orbiter to the icy moon. Until then,
the researchers will have to be content re-examining the images retrieved
by the Voyager and Galileo missions.